My interpretation of Vampires:
They can not syphone mana from the Aether because they are "dead", so they need to get mana from else where, primarily blood because living creatures are the majority when it comes to store of mana in the physical plain and they would be more attracted to humans as food because humans understand more about mana and therefore have more within their blood then let's say, a cow. So basically, they need mana to not decay into dust.